Question

calculate the standard emf for each of the following reactions.
H2(g)+F2(g)→2H+(aq)+2F−(aq).
Cu2+(aq)+Ca(s)→Cu(s)+Ca2+(aq).

Answer 1

In tables, all reaction are reduction reactions

For H : 2 H+ + 2 e- -> H2 E = 0 V (definition)
In first reaction H oxidized, and E = 0 V

Then find E for reaction F2 + 2e -> 2 F-
calculate sum together

In next reaction Copper is reduced and Ca oxidized
Find E for reactions Cu2+ + 2e- -> Cu
Ca2+ + 2e- -> Ca
Because lower reaction happens from right to left, you
have to change sign for E.